(1) Field of the Invention
This invention relates to a double-layer electroplated steel sheet which has excellent properties in corrosion resistance after painting and wet adhesion of paint film.
(2) Description of the Prior Art
Galvanized steel sheets with high corrosion resistance are widely used for materials which are expected to be exposed to highly corrosive environments. Recently, various kinds of galvanized steel sheets are increasingly employed in the automobile industry mainly for interior panels of vehicle bodies to ensure a higher corrosion resistance. In such a case, the galvanized steel sheet is used after coating a primer by electrophoretic painting so that it is usually required to have satisfactory properties in corrosion resistance after painting and adhesion strength of paint film, especially in paint adhesion after immersion in hot water, in addition to uncoated (naked) corrosion resistance. However, the conventional electrogalvanized or hot dip galvanized steel sheets are inferior to cold rolled steel sheets in the wet adhesion of paint film. Galvannealed steel sheets are higher in wet adhesion of paint film than the electrogalvanized or hot dip galvanized steel sheets but are still slightly inferior to cold rolled steel sheets in that property, leaving a problem to be solved before application to exterior panels of vehicle bodies. Therefore, there has been a strong demand for zinc-coated steel sheets with high wet adhesion of paint film comparable to that of cold rolled steel sheets.
The application of galvanized steel sheets to the automobile bodies involves another difficulty in that galvanized sheets with heavy zinc coating weight are low in weldability in spot welding which is frequently resorted to in the automobile industry. The increased use of the galvanized steel sheets is reflected by a greater zinc content in the steel scraps to be recycled, giving rise to a problem in the melting stage of the scraps. In view of these problems or difficulties and for the purpose of cost reduction, there has been a growing demand for galvanized steel sheets which have a light zinc coating weight and yet can ensure a corrosion resistance comparable to that of the conventional couterparts.
In this connection, electroplating of an alloy such as Zn-Ni and Zn-Fe has been attracting special attention. The mono-layer electroplating of Zn-Ni or Zn-Fe has long been known in the art as a means for providing a corrosion resistant coating. The method of such alloy elecroplating and the properties of the plated layer are known, for example, from "Electrodeposition of alloys" by A. Brenner, 1963, Academic Press. Among them, the electrodeposited Zn-Ni alloy has excellent properties especially in corrosion resistance but, similarly to the conventional galvanized steel sheets, it is inferior to the cold rolled steel sheets in wet adhesion of paint film. On the other hand, the electroplated Zn-Fe alloy layer which is considered to have good corrosion resistance particularly in an Fe content range of 7-30% is still inferior to the Zn-Ni alloy electroplating. However, with regard to paintability, it excells pure zinc coating and Zn-Ni electroplating and is comparable to galvannealing.